In a telephone plant the data required for sizing plant requirements and for monitoring the grade of service in its various parts consist of three main types: Usage, congestion, and number of calls. Three different facilities are usually used to collect that data. The facility for usage measurement consists of traffic usage recorders of which various electromechanical and electronic types are available on the market. They can be broadly classified into two main categories, analog and digital. In the analog, the sum of the current drain of busy servers is continuously monitored. In the digital, the servers are scanned at regular intervals and the number of busy servers is scored on digital counters which sum their number during the period of measurement. The measured load representing the mean of the carried traffic, is expressed in convenient units of telephone traffic such as CCS (hundred-call-seconds) or Erlangs (call hours).
Other special measurements are also used for the monitoring of call congestion. These are: peg count, overflow, all trunks busy (ATB) and last trunk busy (LTB). Overflow measurement consists of scoring the number of call attempts which arrive when all servers in the group under consideration are busy. The ratio of such unsuccessful call attempts to all call attempts (which are measured by another counter, sometimes called a peg count register), is called the grade of service. It is used as a measure of congestion. Theoretically, the grade of service is defined as the limit (as time increases without bound) of the ratio of the overflow and all call attempts. In ATB measurement one counter per group of servers measures the duration of the all servers (trunks) busy (ATB) condition. Similarly, in LTB measurement a counter measures the duration of the last server busy condition. The grade of service can then be obtained from that data by mathematical formula.
Telephone traffic consists of calls which arrive at varying times and occupy the servers for varying durations called call holding times. Each call has an arrival and departure time; the average time between successive call arrivals is called the interarrival time. The mean of the load carried by a group of servers can be determined from the integral over time of a staircase function describing the number of occupied servers or occupancy over time. The result of this integral is then divided by the measurement interval for normalization, yielding the average traffic. The mean of the load can also be expressed as the sum of the duration of all calls in an hourly period or as the product of the number of calls and their average holding time.
In digital traffic measuring systems with scanners the system state is monitored by examining the measuring leads and counting the number of occupied servers at each scan. The mean of the carried load is the average number of occupied servers. This gives an estimate of the integral of the staircase function mentioned above; however, it is apparent that changes in the number of occupied servers can occur between successive scans without being detected, thus resulting in measurement inaccuracies. In addition, a large number of measuring leads are required, one for each server.
The device to be described relies on the state transitions or changes in the state of occupancy, rather than repeatedly scanning and counting the number of occupied servers. In order to accomplish this, the number of occupied servers is detected from the state transitions starting with the known initial state, as will be described in the detailed description herein. Because state transitions are used in the device to determine the state of occupancy, it is not necessary to identify which server lead is involved in a particular state transition.
This allows the use of only three measuring leads for an entire group of servers. One lead provides a pulse when any server is seized, another lead provides a pulse when any server is released, and a third initializes the measuring device by providing a pulse when all servers become occupied.
It is believed that this is the first time that a device has been proposed which utilizes seizure and release pulses, together with ATB or congestion pulses as a means for measuring traffic usage without individually scanning the states of all servers. The device also measures the duration of congestion and noncongestion periods to determine the offered traffic from the carried traffic. In addition, the invention described herein overcomes the problem of establishing the initial condition or the initial state of the serving group by initializing the state of the measuring device through the third lead, indicating that all servers are occupied, at which time the exact number of busy servers is known.
This third lead may either provide a pulse when all servers become occupied or at the first instance that a call is blocked after all servers become occupied. The latter condition is presently used in measuring systems to count the number of blocked or overflow calls.
It should be noted that the term "server" as used in the description of this invention is a general term illustrating the range of applicability of the invention. It is believed that the invention would be particularly useful for measurement of traffic in trunks or circuits associated with calls over telecommunication networks, and therefore specific terms used herein refer to these systems.